A cellular communication system projects a number of cells onto the earth. A frequency spectrum is then allocated in frequency, in time, by coding, or a combination of these, to the cells so that communications taking place in nearby cells use different channels to minimize the chances of interference. On the other hand, communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results without interference.
One problem which cellular communications systems address is the handing-off of communications between cells, particularly between cells of different satellites. Relative movement between end users and cells causes the end users and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must "handoff" the call when the end user crosses a cell boundary. If a call is not handed off to a new cell upon leaving an old cell, the call will eventually be lost because the strength of signals over which communications take place would diminish to a point where the system's radio equipment cannot receive the end user's transmissions, or vice versa.
Conventional terrestrial cellular handoff techniques may work adequately when the distances between subscriber units and system transceivers are relatively small, when speeds of movement between cells and subscriber units are slow, and when handoffs are relatively evenly distributed in time. Such conditions are present for conventional terrestrial cellular systems in which cells do not significantly move with respect to the earth and the movement between cells and subscriber units results from subscriber movement in accordance with conventional modes of transportation. On the other hand, when system radio equipment is located on satellites orbiting the earth in moving orbits, these conditions are not present, and the conventional handoff techniques may be inadequate.
For example, orbiting satellites are located a relatively large distance from subscriber units, often on the order of several hundred kilometers. The smaller this distance, the greater the speed of the satellite relative to a particular position on the earth. Speeds of over 20,000 km/hr are typical. This fast movement relative to a subscriber unit introduces widely and rapidly varying propagation delays and Doppler frequency offsets into signals transmitted between a satellite and a subscriber unit. The widely and rapidly varying propagation delays and Doppler frequency offsets make handoffs involving different satellites more complex and time consuming. The resynchronization process required to generate accurate time delay parameters and Doppler frequency offset parameters compatible with the newly servicing satellite may be time consuming and complex.
Accordingly, there is a significant need for an inter-satellite handoff method and system, including a subscriber unit, which permit direct generation and calculation of handoff parameters, permitting an efficient handoff directly to a traffic channel of a new servicing satellite.